1. Field of the Invention
The present invention relates to a method of centering a disk for a disk drive on the hub of a disk drive motor and an apparatus for performing disk centering.
2. Description of the Related Art
Motors in disk drives usually spin the disk mounted on the motor at approximately 3600 rpm. The sensitivity of disk drives to vibration requires that the disk and motor assembly be balanced so that the motor assembly does not vibrate beyond a specified degree during operation of the disk drive. The disk and motor are usually balanced with weighted screws threaded into the motor or by using other weighting methods.
FIG. 1 illustrates a disk drive motor 10 having a hub 12. Hub 12 has a first hub portion 13.sub.1 which protrudes into the center hole of disk 14 and a second first hub portion 13.sub.2 which protrudes through the center hole of disk 14 and through disk clamp 16. Disk 14 is secured to motor 10 by disk clamp 16 and screws 18.sub.1-2. Disk 14 has a ring-like shape with an outer diameter 20 and an inner diameter 22; inner diameter 22 defines the center hole of disk 14.
If weighted screws are to be added for balancing, more holes than are necessary to hold disk 14 in place are provided in the hub 12 of motor 10 and the weighted screws are threaded into the extra holes to balance disk 14. In addition, screws 18, which are usually all of the same weight, may be replaced with screws of varying weights during the balancing process. As used herein, "hub" means the rotating portion of a disk drive motor. The balancing procedure is conventionally performed by spinning the disk, detecting an out-of-balance condition, providing screw(s) of the appropriate weight in the appropriate hole(s) in the hub, and repeating the procedure until the disk is balanced. Alternatively, weight can be added in different manners (e.g., by placing lead tape on the hub 12 or disk 14).
Several problems are associated with the conventional balancing procedure. First, the balancing procedure is time and labor intensive, and extremely difficult to automate. Second, the efforts associated with the balancing procedure are often wasted because of poor disk placement relative to the hub 12, particularly first hub portion 13.sub.1 which protrudes into the center hole in disk 14. If the disk 14 is not centered on the hub 12, a portion of inner diameter 22 of disk 14 is closer to first hub portion 13.sub.1 than the remaining portions of inner diameter 22; in some cases a portion of inner diameter 22 may even contact first hub portion 13.sub.1. The clearance between inner diameter 22 of disk 14 and first hub portion 13.sub.1 is on the order of 0.0004 to 0.008 inches, and therefore manually locating disk 14 to prevent contact with first hub portion 13.sub.1 is difficult if not impossible. The problems associated with centering the outer diameter 20 of the disk 14 relative to the axis 11 of motor 10 are compounded by runout of motor 10 and non-concentricity of outer diameter 20 and inner diameter 22 of disk 14.
Thermal expansion of hub 12 and/or thermal contraction of disk 14 will cause a portion of hub 12 and disk 14 to contact one another, if they are not already in contact. Contact between disk 14 and first hub portion 13.sub.1 causes disk 14 to move relative to hub 12, placing disk 14 in an out-of-balance condition. This thermal expansion/contraction problem is enhanced by the different coefficients of thermal expansion of the disk material and the hub material, e.g., the aluminum-based disk and steel-based motor hub, and the difficulty in placing disk 14 on hub 12 without contact between first hub portion 13.sub.1 and disk 14.